This invention relates generally to modulators for television signal scrambling systems and specifically to modulators for scrambling systems that utilize suppressed synchronizing signals.
It is well known for cable operators to scramble television (TV) signals to prevent unauthorized utilization of these signals. In a common form of scrambling, the horizontal synchonizing pulses (sync pulses) in the TV signal are suppressed except during the vertical interval. When the scrambled baseband video signal is modulated onto the channel frequency carrier, difficulty is encountered. The modulator is designed to clamp to the sync tip level to maintain the correct video level in the signal. The horizontal sync pulses during the vertical interval are at the proper level and enable the modulator to clamp properly. The clamp begins to drift, however, in the presence of the suppressed sync tips and may clamp onto the video signal. The net result is that, unless corrected, the scrambled video signal upplied to the modulator will be distorted and result in a distorted video signal being transmitted. While circuits in the prior art have been developed for correcting this deficiency, they are expensive and cumbersome.
In the present invention, an automatic clamp circuit arrangement splits the scrambled video signal that is being supplied to the modulator into two video signals. In one video signal, the horizontal sync tips are restored to their proper level and that signal is supplied to the clamp for controlling the video level of the input amplifier. Thus, the output of the amplifier is clamped at the proper sync tip level despite the fact that the sync tips in the scrambled video signal are suppressed.